The default android telephony app allows the configuration of SIP accounts. At home, I want to use my android phone as a softphone with my local router (an AVM fritzbox). However I frequently use other Wifi networks. As far as I understand, with a SIP account setup to accept incoming calls, the phone will now try to connect to my home router whatever network it is connected to, even using mobile internet services, using up bandwidth. I know that this can be restricted using dedicated SIP apps (like the one AVM provides itself). But it is it also possible to only search for the SIP server when connected to a given network using just the native telephony app?

You didn't specify the Android version of your device. With 4.0+, you might want to check with your data monitor whether you can find the native app, and if so, set the background data limit to 0 for it. That should prevent it from looking for the SIP server at least when running on mobile data.
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IzzyMay 1 '13 at 20:02